Human immunodeficiency virus type 1 (HIV-1) leads to the contraction of acquired immune deficiency disease (AIDS). The number of cases of HIV continues to rise, and currently over twenty-five million individuals worldwide suffer from the virus. Presently, long-term suppression of viral replication with antiretroviral drugs is the only option for treating HIV-1 infection. Indeed, the U.S. Food and Drug Administration has approved twenty-five drugs over six different inhibitor classes, which have been shown to greatly increase patient survival and quality of life. However, additional therapies are still required due to a number of issues including but not limited to undesirable drug-drug interactions; drug-food interactions; non-adherence to therapy; drug resistance due to mutation of the enzyme target; and inflammation related to the immunologic damage caused by the HIV infection.
Currently, almost all HIV positive patients are treated with therapeutic regimens of antiretroviral drug combinations termed, highly active antiretroviral therapy (“HAART”). However, HAART therapies are often complex because a combination of different drugs must be administered often daily to the patient to avoid the rapid emergence of drug-resistant HIV-1 variants. Despite the positive impact of HAART on patient survival, drug resistance can still occur and the survival and quality of life are not normalized as compared to uninfected persons.1 Indeed, the incidence of several non-AIDS morbidities and mortalities, such as cardiovascular disease, frailty, and neurocognitive impairment, are increased in HAART-suppressed, HIV-infected subjects.2 This increased incidence of non-AIDS morbidity/mortality occurs in the context of, and is potentially caused by, elevated systemic inflammation related to the immunologic damage caused by HIV infection.3,4,5 
Sustained successful treatment of the HIV-1-infected patient population with drugs will therefore require the continued development of new and improved drugs with new targets and mechanisms of action including antiretroviral and/or interventions aimed at restoration of the immune system and decreasing the systemic inflammation.
IDO is a monomeric 45 kDa extrahepatic heme-containing dioxygenase which catalyzes the oxidative pyrrole ring cleavage reaction of I-Trp to N-formylkynurenine utilizing molecular oxygen or reactive oxygen species via three proposed reaction mechanisms.6 IDO is an enzyme that is the rate limiting step in the kynurenine pathway of tryptophan catabolism. IDO catalyzes the dioxidation of the indole ring of tryptophan (Trp), producing N-formyl-lynurenine (NFK), which is then metabolized by other enzymes into several downstream metabolites such as kynurenine (Kyn) and 3-hydroxy-anthranilate (HAA). The depletion of Trp and accumulation of Kyn and HAA have immunomodulatory activity, typically exemplified by decreased T cell activation and proliferation, enrichment of regulatory CD4+ T cells, and depletion of IL-17-producing CD4+ T cells. IDO activity therefore has a general immunosuppressive impact.
IDO is expressed in response to inflammation and is considered an important counter balance to prevent collateral tissue damaged during prolonged inflammation. IDO expression and activity are elevated during chronic viral infections such as HIV and HCV, chronic bacterial infections, as well as acute conditions such as sepsis. The IDO-mediated shift of Th17 to Treg differentiation of helper T cells likely plays a role in the intestinal immune dysfunction during HIV infection, likely related to the observed elevated systemic inflammation and increased incidence of non-AIDS morbidity/mortality. In addition, IDO activity likely also plays a role in the persistence of pathogens and cancer, and inhibition of IDO may improve clearance mechanism, potentially leading to cure of these chronic diseases. IDO may also play a role in neurological or neuropsychiatric diseases or disorders such as depression by modulating serotonin synthesis or production of excitatory neurotoxins such as kynurenine. As such, pharmacologic inhibition of IDO has application in a broad range of applications from neurology, oncology, and infectious diseases.
It would therefore be an advance in the art to discover IDO inhibitors that effective the balance of the aforementioned properties as a disease modifying therapy in chronic HIV infections to decrease the incidence of non-AIDS morbidity/mortality; and/or an immunotherapy to enhance the immune response to HIV, HBV, HCV and other chronic viral infections, chronic bacterial infections, chronic fungal infections, and to tumors; and/or for the treatment of depression or other neurological/neuropsychiatric disorders.